The open end of canbodies is commonly reduced in diameter and flanged. The flange facilitates attaching a closure to the end and the reduction in diameter allows using a smaller closure thereby saving material. Furthermore, reducing the diameter does not substantially decrease the volume of the can.
The beading of cans in a production line can manufacturing facility generally requires a multiple step operation. One or more reductions in diameter are achieved by forcing the open end of the can into an inwardly tapered necking die until the appropriate amount of plastic deformation occurs. The terminal edge of the can is then outwardly flanged to an increased diameter.
A major source of defective cans, the split flange, often results from this necking and flanging operation. It is to be appreciated that in the formation of the canbody a substantial amount of strain hardening takes place. When the end is then reduced in diameter, the ductility of the metal is further reduced and wrinkling or buckling may occur. During the following flanging operation split flanges may occur due to the high tensile forces exerted circumferentially on the strain hardened neck by the flanging tool and due to existing wrinkles in the neck which may act as stress concentrators.
One method of reducing the amount of work that the flanged end is subjected to is a process that will broadly be referred to herein as "spin-necking." In spin-necking the canbody is rotated while inner and outer forming rolls neck and flange the end portion. Unlike a conventional production line operation where the entire end portion of the can is reduced in diameter (necked) and the terminal edge is then flanged to an increased diameter, in spin-necking the terminal edge is supported by the inner forming roll and flanged from its original diameter without first being reduced in diameter. Therefore, the work involved in reducing the terminal edge by necking and then flanging back to the original diameter is avoided. This greatly reduces the risk of split flanges, both at the can manufacturing facility and at the beverage filling facility where the closure is applied.
Various approaches to spin-necking are taught by U.S. Pat. No. 3,967,488 to Hasselbeck et al., U.S. Pat. No. 4,176,536 to Parknin et al., and U.S. Pat. No. 4,070,888 to Laszlo. Laszlo discloses an apparatus and method for spin-necking canbodies which utilize two independent annular forming surfaces which move in opposite axial directions during the necking process. Both Hasselbeck and Parknin utilize inner support tooling which extends over a major portion of the length of the canbody. Hasselbeck uses an internal gripping means which presents an uninterrupted surface to the inner peripheral surface of the can. Although such a gripping means reduces the chance of wrinkling and buckling it also poses substantial practical problems in a production line situation. Parknin teaches a highly complex apparatus that has some of the drawbacks of Hasselbeck due to the large internal tooling used.
Generally, in a production line, cans which are to be beaded have already had a protective coating applied to their interior surfaces. This coating protects the contents of the can against the absorption of a metallic taste and more importantly, with some corrosive soft drink beverages, protects the metal can from the corrosive influence of the beverage. It is therefore imperative that the integrity of this coating be preserved throughout the beading operation. Also, in many situations, no further cleaning of the canbody will be done after necking and flanging and prior to filling. Therefore, any contact with the interior surface of the canbody will increase the risk of contamination with foreign matter which may be carried by the contacting member. Hence, large internal tooling in a necking operation in undesirable for contact with the interior surface of the canbody should be avoided to the greatest extent possible.
Furthermore, positive gripping means, such as those taught by Hasselbeck in the aforementioned patent, are generally incapable of handling canbodies having even small variations in diameter. The gripping means must be designed for a single size of can within very low tolerances. If a can is too large, the positive gripping means is ineffective. If the can is too small, damage to the can may occur when the gripping means is fully expanded. In either case, there is also a greater chance of damage to the interior protective coating. Unfortunately, in a can manufacturing facility, due to variations in metal stock, temperature and tool wear, a relatively wide range of diameters are produced. To decrease the tolerance range within which cans have heretofore been acceptable would be prohibitively expensive. For the above and other reasons, the search for an improved method of spin-necking has continued.